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So, my first week on the bench turned out to be Fender Hot Rod Deluxe & Devilles as far as the eye can see. But one of the jobs has me scratching my head and I'm hoping maybe you guys can help me out with this one.
Customer brings in the amp for general service, bias, cleaning, etc. I don't have the work order in front of me right now, but I believe he may have brought it in for specific issue. It seems every hot rod that I worked on had a plate resistor failure, and at least on of the screen resistors had already been replaced. I remember replacing the 100k plate load resistor in his amp. Anyway, he needed new output tubes and he told us to replace all the preamp tube as well. After we were done with the service and cleaning, it sounded good, looked like new, and he couldn't have been happier.
The next day he walks in with the amp because, he said after 3 hours of playing, it started to sound terrible (and it did). Kind of like that weak power accompanied by scratchy distortion which almost sounds like digital artifacts as a note fades away.
Right away I though, "this sounds a lot like a failed plate load resistor in the phase inverter. What are the odds that the 82k failed right after swapping out the 100k?" Obviously, I it couldn't have been the one I did the work on
But, when we sine wave tested it, the output looked really terrible and more like static than a sine wave.
Another tech had it on his bench and was testing some of the solder joints in the power supply filtering. When tapping on the boards, there was evidence that the waveform on the scope would momentarily correct itself. We were suspicious of the solder joints, and opted to pull the board to reflow the solder on the caps. The customer had already paid a large bill, and rather than reflow solder on the original IL caps which were there, we replaced them with new ones.
Now, here is where the weird shit started to go down. With the tubes pulled, we checked the voltages in the B+ and bias. Everything checked out. We reinstalled the tubes to check the bias and sine wave test it again. But, when the tubes heated up, the bias voltage started climbing... fast! from -51V(or whatever) to -20,-10,+10,+30/60/120. Kill the power, the tubes are red-plating...
Okay, that is F'd up. The voltages would all test fine until the tubes would heat up and draw current and the then bias would skyrocket in a hurry. My initial though, was that the board had become conductive and was leaking HV. But, that wasn't the case. It turns out, we isolated it to the new JJ 12AX7 we put in the phase inverter, had completely failed. We swapped it out to be sure, and a new tube fixed the problem. No sign of bias drift. We tested it several times, left it on, and played it to be sure. But, a new tube fixed the problem. What is bothering me though, is, I'm looking at the schematic and I don't understand how a failed tube in the phase inverter could cause that. Can you guys shed some light on this?
Customer brings in the amp for general service, bias, cleaning, etc. I don't have the work order in front of me right now, but I believe he may have brought it in for specific issue. It seems every hot rod that I worked on had a plate resistor failure, and at least on of the screen resistors had already been replaced. I remember replacing the 100k plate load resistor in his amp. Anyway, he needed new output tubes and he told us to replace all the preamp tube as well. After we were done with the service and cleaning, it sounded good, looked like new, and he couldn't have been happier.
The next day he walks in with the amp because, he said after 3 hours of playing, it started to sound terrible (and it did). Kind of like that weak power accompanied by scratchy distortion which almost sounds like digital artifacts as a note fades away.
Right away I though, "this sounds a lot like a failed plate load resistor in the phase inverter. What are the odds that the 82k failed right after swapping out the 100k?" Obviously, I it couldn't have been the one I did the work on
But, when we sine wave tested it, the output looked really terrible and more like static than a sine wave.
Another tech had it on his bench and was testing some of the solder joints in the power supply filtering. When tapping on the boards, there was evidence that the waveform on the scope would momentarily correct itself. We were suspicious of the solder joints, and opted to pull the board to reflow the solder on the caps. The customer had already paid a large bill, and rather than reflow solder on the original IL caps which were there, we replaced them with new ones.
Now, here is where the weird shit started to go down. With the tubes pulled, we checked the voltages in the B+ and bias. Everything checked out. We reinstalled the tubes to check the bias and sine wave test it again. But, when the tubes heated up, the bias voltage started climbing... fast! from -51V(or whatever) to -20,-10,+10,+30/60/120. Kill the power, the tubes are red-plating...
Okay, that is F'd up. The voltages would all test fine until the tubes would heat up and draw current and the then bias would skyrocket in a hurry. My initial though, was that the board had become conductive and was leaking HV. But, that wasn't the case. It turns out, we isolated it to the new JJ 12AX7 we put in the phase inverter, had completely failed. We swapped it out to be sure, and a new tube fixed the problem. No sign of bias drift. We tested it several times, left it on, and played it to be sure. But, a new tube fixed the problem. What is bothering me though, is, I'm looking at the schematic and I don't understand how a failed tube in the phase inverter could cause that. Can you guys shed some light on this?
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